Compass calabration

[thaeni]

Should regular ship compass agree with fluxgate compass used
with auto pilot?. Both are magnetic and I assume they should agree,
but I've been on quite a few boats and haven't found a matching pare
yet.

 

[Rich Stidger]

In a perfect world, yes they should agree.

However the ship compass and the fluxgate compass are probably mounted in different locations and they are each subject to different deviation forces.

I would be quite surprised if you found these compasses to agree on any boat.

 

[thaeni]

Then which compass is the Bible?

 

[Rich Stidger]

IMHO, if you want an accurate compass, you need to produce a deviation chart for the compass of choice. Given this accurate deviation chart you can correct your compass for any direction that it shows.

If you have a "good" compass installation that a deviation chart shows low values for the entire 360 degrees, you can generally ignore the deviation unless you are on a long sea voyage. In other words, an error of 1-2 degrees will not produce a large off-course error over a common run of 5-10 miles. Besides, 1-2 degree error is probably within the error of your ability to steer.

Now if you only have a compass and no other positioning equipment like GPS or radar, you had better pay attention to the deviation table in all cases and also hone your helmsman skills.

While the fluxgate compass also has a deviation, it is controlling the autopilot which is a steering aid, not a competent helmsman. I presume that if the autopilot was off by a half-mile or so you would make the necessary correction as you approached your waypoint or destination.

 

[n624ma]

The magnetic compass is king!

The Ships Compass (aka Steering Compass, Binnacle Compass, etc) should be the "Master" reference. It can (and should) be "Swung" (adjusted) to remove as much deviation as possible.

The flux-gate sensor for the autopilot is usually "compensated" by a program in the autopilot software. If it is a few degrees different from the steering compass it really doesn't matter, the autopilot will steer whatever direction it is pointed when engaged.

 

[Bill Roosa]

The one that reports accurately
ALL compasses read "wrong" The magnetic pole being located away from the north pole sees to that.
ALL compasses should have a deviation card so you can interpret their output with a true direction.

 

[sandpiper10471]

Rely on the magnetic compass. Fluxgates require electricity and one never knows when that fact might become problematic.
I never set my autopilot to the required course. I set my course first then ask the pilot to go there. Point and shoot. Then as the destination gets closer, I make adjustments.

 

[Rick486]

A man with two watches has no idea what time it is.

Steer with the ship's compass, making sure it is adjusted properly, then update your heading based on ship's position, as determined by GPS or other means. Regardless of compass errors, our boats make leeway and are affected by currents and those effects must be compensated in any case.

 

[Dave602]

Check your books for the Autohelm. My ST4000 has a procedure to calibrate the compass heading by turning a large slow circle in calibration mode. It will then display the deviation it picked up. If it is too big of deviation you may have to move the compass. It will also flash the heading. You can then use your + and - heading to make it agree with your magnetic compass heading.